Shell & Tube Heat Exchanger

A waste heat recovery boiler manufactured
by Missouri Boiler and Tank Co. was used in the recovery of heat from
process steam in an chemical processing plant. In this recovery
boiler, there are 166 tubes making two passes inside the shell. The
process steam enters the recovery boiler at 260°C and leaves at
200°C as a condensate in the tubeside. In the shellside, a
subcooled water at 50°C enters the shell and leaves at
193°C as a superheated steam.

The outside walls of the tubes regularly
scaled up. This resulted in a loss of heat transfer, leading to the
reduction of condensate production. This necessitated plant shut down
for maintenance. This particular boiler was located on the second
floor of an exterior tower, hence a crane was needed to remove the
bundle. This procedure required eight men, three days of scraping
sand blasting and jet spraying to remove the scales from the tube
bundles.

On one occasion, the bundle became so
encrusted with scale that it become completely wedged in the boiler
and was damaged when removed for cleaning. This resulted in a two
week shutdown and a new, $50,000 bundle.

A hot water boiler was used to provide
domestic hot water from waste steam in a semiconductor plant in a
hard water area in Pennsylvania. Due to health-related concerns, the
plant decided not to use any scale-inhibiting chemicals even though
they had a very hard water of 350 ppm CaCO3 equivalent. After one
month operation without ED2000 unit, a new tube bundle was completely
coated with thick scales. Subsequently, the overall heat transfer
coefficient was dropped by 70% from the maximum value.

Solution:

The Electronic Descaling 2000 unit was
installed on the water inlet to the steam water heater. After one
month operation, the new tube bundle was removed and found
clean.